1. Field of the Invention
The present invention relates to a hydraulic transmission and, more particularly, to a hydraulic transmission suitable for transmitting the rotational power of an input shaft to an output shaft at a varying speed.
2. Description of the Prior Art
In the prior art, a hydraulic motor is rotationally driven to extract a rotational power therefrom by inputting the rotational power to a hydraulic pump to feed the pressurized oil discharged from the hydraulic pump to the hydraulic motor. The rotating speed of the hydraulic motor is varied by using either of a hydraulic pump or motor of variable capacity type.
With this structure, however, power loss is increased because the entire power inputted to the hydraulic pump is transmitted through the pressurized oil to the hydraulic motor. The hydraulic pump and motor are large-sized and expensive because they have to be designed to have capacities sufficient for the maximum power to be transmitted.
In order to solve these difficulties, there has been proposed a hydraulic transmission which has its power transmission efficiency improved by dividing the power trains into mechanical and hydraulic transmissions to increase the ratio of the power mechanically transmitted under a high load.
The hydraulic transmission of this type is exemplified in FIGS. 15 to 17.
In FIG. 15, reference numeral 01 designates a shaft which is retained irrotationly in a stationary member 03 through a coupling 02 splined to the right-hand end thereof, and which has its lefthand end splined to a cylinder barrel 04. This cylinder barrel 04 is formed with a plurality of (e.g., seven, as shown) cylinders 05 which extend from the righthand end face thereof in the axial direction and at a predetermined spacing in the circumferential direction of a cylindrical plane formed on the axis thereof. Pistons 06 are fitted liquid-tight and slidably in the cylinders 05, respectively. Each of those pistons 06 has its outer end ball 06a received in a slipper pad 07 while being permitted to accomplish relative angular movements. The slipper pad 07 is held by a retainer 08 in sliding contact with the surface of a liner 010 which is fixed on a swash plate 09. This swash plate 09 is rotatably supported on a trunnion 011 perpendicular to the shaft 01. A slider 012 extends radially from the periphery of the swash plate 09 and is fitted slidably in a groove 013a of a ring-shaped guide 013.
Thus, the swash plate 09 can be rotated around the trunnion 011 by moving the guide 013 toward the shaft 01 to change the angle of inclination of the liner 010, i.e., the inclined sliding face to an arbitrary value.
The cylinder barrel 04 is opened, as shown in FIG. 16, by cocoon-shaped cylinder ports 014 which communicate with the cylinders 05, respectively. These cylinder ports 014 are aligned, as shown in FIG. 17, with a crescent high-pressure port 016 or low-pressure port 017, which is opened into the righthand end face of a valve block 015.
Reference numeral 018 designates an output shaft which is arranged coaxially with the shaft 01. To the righthand end of the output shaft 018, there is splined a cylinder barrel 019. This cylinder barrel 019 is formed with a plurality of (e.g., seven, as shown) cylinders 021 which extend axially from the lefthand end face thereof and at a predetermined spacing in the circumferential direction of a cylindrical face formed around the axis of the cylinder barrel 019. Pistons 022 are fitted liquid-tight and slidably in the cylinders 021, respectively. Each of these pistons 022 has its outer end ball 022a received in a slipper pad 023 while being permitted to accomplish relative angular movements. The slipper pad 023 is so retained by a retainer 024 such that it is held in sliding contact with the surface of a liner 026 fixed on a swash plate 025 united with a lefthand internal casing 034. The cylinder barrel 019 has its righthand end face opened by a cocoon-shaped cylinder port 027 communicating with each cylinder 021 like that shown in FIG. 16. The cylinder ports 027 are aligned with a crescent high-pressure port 028 or low-pressure port 029 opened into the lefthand end face of the valve block 015 like that shown in FIG. 17. Moreover, this high-pressure port 028 is made to communicate with the high-pressure port 016 by way of a high-pressure liquid passage 30, whereas the low-pressure port 029 is made to communicate with the low-pressure port 017 through a low-pressure liquid passage 031. The cylinder barrel 04 is supported by a bearing 032 in the rightward projecting end of a rod 051, which extends through the center of and is fixed by the valve block 015. The cylinder barrel 019 is supported by a bearing 020 in the leftward projecting end of the rod 051.
The valve block 015 is interposed at its peripheral edge between the lefthand end face of a righthand internal casing 033 and the righthand end face of a lefthand internal casing 034, and these members are fastened together by means of bolts 035. The shaft 01 extends through a cylindrical portion 033a extending rightward from the righthand end of the righthand internal casing 033. The cylindrical portion 033a is supported on the shaft 01 by a bearing 036 arranged in the cylindrical portion 033a. The clearance between the cylindrical portion 033a and the shaft 01 is sealed up by a sealing device 037 which is arranged at the righthand side of the bearing 036. Moreover, a pinion 038 is keyed at 052 to the righthand periphery of the cylindrical portion 033a and is retained on the cylindrical portion 033a by a nut 039 screwed on the righthand end of the cylindrical portion 033a. On the other hand, the output shaft 018 extends through a cylindrical portion 034a which extends leftward from the lefthand end of the lefthand internal casing 034. The cylindrical portion 034a is supported on the output shaft 018 through a bearing 040 which is arranged in the cylindrical portion 034a . The clearance between the cylindrical portion 034a and the output shaft 018 is sealed up by a sealing device 041 which is arranged at the lefthand side of the bearing 040.
The trunnion shaft 011 has its two ends supported in the righthand internal casing 033. Reference numeral 042 designates an external casing which is fixed to the stationary member 03 by conventional means (not shown). The external casing 042 is constructed by fastening a cylindrical member 043 and an end plate 044 covering the righthand opening of the cylindrical member 043 by means of bolts 045. The cylindrical member 043 has its lefthand end portion supported on the cylindrical portion 034a of the lefthand internal casing 034 by means of a bearing 046. The clearance between the cylindrical member 043 and the cylindrical portion 034a is sealed up by a sealing device 047 which is arranged at the lefthand side of the cylindrical portion 034a. On the other hand, the end plate 044 is supported on the cylindrical portion 033a of the righthand internal casing 033 by means of a bearing 048, and the clearance between the end plate 044 and the cylindrical portion 033a is sealed up by means of a sealing device 049 which is arranged at the righthand side of the bearing 048.
Thus, when the pinion 038 is rotationally driven, it rotates the righthand internal casing 033, the lefthand internal casing 034, the valve block 015 and the swash plates 09 and 025 together. In accordance with these rotations, the valve block 015 is rotated relative to the cylinder barrels 04 and 019, while having its righthand end face held in sliding contact with the lefthand end face of the cylinder barrel 04 and its lefthand end face held in sliding contact with the righthand end face of the cylinder barrel 019. The slipper pads 07 slide on the inclined sliding face of the liner 010 which is fixed on the swash plate 09, and the slipper pads 023 slide on the inclined sliding face of the liner 026 fixed on the swash plate 025.
Thus, while the pistons 06 are reciprocated axially in the cylinders 05 so that the cylinder ports 014 are aligned with the crescent high-pressure port 016, the pistons 06 accomplish their forward strokes to discharge the liquid from the cylinders. While the cylinder ports 014 are aligned with the low-pressure port 017, the pistons 06 accomplish their backward strokes to suck the liquid into the cylinders 05.
The high-pressure liquid discharged from the cylinders 05 flows into the cylinders 021 via the cylinder ports 014, the high-pressure port 016, the highpressure liquid passage 030, the high-pressure port 028 and the cylinder ports 027 to push the pistons 022. Then, the pistons 022 start their backward strokes so that the cylinder barrel 019 and the output shaft 018 splined thereto are rotated because the outer end balls 022a of the pistons 022 are held in sliding contact with the inclined sliding face of the liner 026 fixed on the swash plate 025 through the slipper pads 023.
When the pistons 022 end their backward strokes to pass over their bottom dead centers, they start their forward strokes to discharge the low-pressure liquid from the cylinders 021. The low-pressure liquid thus discharged is sucked into the cylinders 05 via the cylinder ports 027, the low-pressure port 029, the lowpressure liquid passage 031, the low-pressure port 017 and the cylinder ports 014.
Now, if the number of revolutions of the pinion 038 is denoted at n.sub.1, the number of revolutions of the output shaft 018 at n.sub.2, the stroke volume of the cylinders 05 at V.sub.1 and the stroke volume of the cylinders 021 at v.sub.2, the following equation holds: EQU n.sub.2 =n.sub.1 .multidot.(1-v.sub.1 /v.sub.2).
The stroke capacity v.sub.2 is constant because the angle of inclination of the swash plate 025 cannot be changed, but the stroke capacity v.sub.1 can be arbitrarily varied if the swash plate 09 is inclined around the trunnion 011 to change its angle of inclination by moving the guide 013 along the shaft 01. As a result, the number of revolutions n.sub.2 of the output shaft 018 can be arbitrarily changed.
If, on the other hand, the angle of inclination of the swash plate 09 is set at zero, the pistons 06 are not reciprocated so that the stroke capacity v.sub.1 is zero. Then, the pistons 022 are not reciprocated if the liquid does not leak. As a result, the cylinder barrel 019 is rotated at the same speed as that of pinion 038 as if it were locked by the swash plate 025 and the valve block 015. Thus, the power inputted from the pinion 038 is mechanically transmitted to the output shaft 018 through the righthand internal casing 033, the valve block 015, the lefthand internal casing 034 and the cylinder barrel 019.
In this hydraulic transmission, the valve block 015 is rotated, while sliding, relative to the cylinder barrels 04 and 019. Thus, this transmission is susceptible not only to friction loss at those sliding faces, but also to wear of the sliding faces and liquid leakage from the sliding faces.
Since, moreover, the swash plate 09 is rotatably supported in the righthand internal casing 033 through the trunnion 011, it is rotated by rotations of the pinion 038 so that dynamic imbalance is seriously increased.
Since, moreover, high frequency vibrations of swash plate 09 accompany the reciprocations of the pistons 06 it is difficult to maintain the reliability of the slider 012 and the guide 013.
Since, moreover, the shaft 01 has to be irrotationally retained, the power has to be inputted from the drive shaft eccentric from the axis of the shaft 01 to the pinion 038 through a gear train or chain.
Since, moreover, the casing has to be double walled, the structure is so complicated and large-sized that the hydraulic transmission is not practical.